But he missed a prime opportunity to win back listeners and earn their forgiveness.

According to Chris Hadnagy, a body language expert and the author of "Unmasking the Social Engineer," Zuckerberg didn't show any signs of sadness in either his facial expressions or his body language. Instead, Hadnagy said, Zuckerberg looked "stoic," "controlled," and relatively emotionless.

Hadnagy has worked with Paul Ekman, a psychologist known for his research on "microexpressions," or the subtle movement of facial muscles that communicate different emotions. Hadnagy said Zuckerberg's stoicism, his stick-to-the-facts approach, and his relative lack of emotion would be more appropriate if he were trying to win an argument.

But given that Zuckerberg was testifying about a mistake Facebook may have made, it would have made more sense for him to show some remorse, Hadnagy said. That's because sadness typically evokes observers' sympathy and empathy. "They let us co-feel emotion," Hadnagy added.

At certain points during the testimony, Zuckerberg looked angry or anxious — emotions that are to be expected in such a stressful circumstance. Yet he never displayed any sadness.

For instance, Hadnagy pointed out that he seemed anxious, with a slight frown, while Senator Feinstein was asking him why Facebook didn't ban Cambridge Analytica in 2015 on Day 1 of the testimony.

And he looked relaxed, his jaw loosened, while addressing Feinstein: "That's a great question. Cambridge Analytica wasn't using our services in 2015, as we can tell."

On Day 2 of the testimony, Hadnagy noted that Zuckerberg showed contempt when asked whether Facebook had an ethical obligation to notify millions of users of the data breach. His jaw is tight and his lips are pursed.